The OPG process is described by the energy conservation statement ℏωP=ℏωS+ℏωI where ωP corresponds to the energy of a pump photon, which splits into two photons called the signal and idler with frequencies ωS and ωI respectively. The conversion efficiency from the pump to signal and idler depends on the nonlinearity of the frequency conversion crystal and to the intensity of the pump beam. Typically the OPG process is very weak, especially for continuous-wave (cw) situations. In these cases a resonator may be placed around the crystal to resonate the signal and in some cases the idler as well. Such a set up is called an optical parametric oscillator (OPO). For Q-switched lasers having pulse durations on the order of 10 nsec or less, the peak intensity is high enough to achieve high conversion efficiency on a single pass of the pump through the nonlinear crystal. When the OPG efficiency is high, it is preferable to the OPO because of the reduced number of optical elements and perhaps more importantly because of the reduced operational complexity. Since the OPG process is single pass, the dynamics of an external OPO cavity are eliminated. Notably, tuning an OPG in a seamless fashion is simpler than an OPO because the OPG is free to oscillate at any frequency whereas the OPO is restricted to specific frequencies defined by the optical cavity modes. An OPG may exhibit similar behavior to an OPO if its end-faces reflect or scatter the signal or idler, which results in a low-finesse, albeit unintentional cavity.
The free-running OPG output bandwidth is typically too large for many applications. Narrowing the linewidth has been accomplished by injection seeding with a filtered OPG (as disclosed in U.S. Pat. No. 6,359,914) or injection seeding with a diode laser. Both techniques work, but the filtered OPG has a subtle advantage. In most cases it is not possible to coat the end facets of the crystal with perfect anti-reflection coatings for the pump, signal and idler. Some residual reflection or scattering leads to a low-finesses cavity. When injection seeding with a diode laser, unless the diode laser matches the residual cavity mode, that seeding is ineffective. This is typically an issue when the pump duration is about 10 nsec where the pump bandwidth is narrower than the free-spectral range (FSR) of the low-finesse cavity. The filtered seed approach alleviates this issue because the filter function is not as sharp as a narrow-linewidth diode laser. The bandwidth of the filtered seed then overlaps one of the cavity modes and seeding is effective.
In cases where the pump duration is about 1 nsec, the pulse has a correspondingly larger bandwidth that (even transform limited) becomes larger than the low-finesses FSR. The signal and idler have similar bandwidths. Since the seed is gated by the pump pulse, the seeding bandwidth is also increased. Seeding in this regime is guaranteed to overlap with at least one of the cavity modes. In this regime a diode laser and filtered signal both are effective seed sources.
The filtered signal approach has a further advantage, it is possible to generate signal over the entire bandwidth of the nonlinear crystal. This is especially important for new classes of nonlinear crystals operating in regions where diode lasers are not available. An example is orientation-patterned gallium arsenide, which operates in the mid-wave infrared region. Although diode lasers are available in the telecommunications region, out to about 1.6 μm, beyond 2 μm direct laser sources are typically expensive or don't exist.
U.S. Pat. No. 6,359,914, which is incorporated by reference herein in its entirety, is based on the filtered signal approach, but requires two nonlinear crystals to operate in most of the embodiments. The two-crystal requirement increases the cost of the system and its complexity. For example, broad tuning of the device requires that both crystals be tuned simultaneously. This may be accomplished by simultaneously temperature tuning both crystals. In the case of quasi-phase matched crystals such as periodically poled lithium niobate (PPLN), tuning may also be accomplished by simultaneously changing the grating periodicity by translating the crystal. As illustrated in FIG. 1, a narrow bandwidth seed is generated by filtering the output of a first stage optical parametric generator. The seed downstream of the filter is amplified in a second stage optical parametric amplifier.
In a separate embodiment of U.S. Pat. No. 6,359,914, a single crystal design is employed. In this embodiment, the pump is split into two beams, one for passing through the crystal in the forward direction to generate a signal and a second pump for amplifying the filtered seed in the backwards direction. This method of using a single crystal by splitting the pump into two beams is different from the current application, which sends the entire pump through the crystal on each pass, and in one embodiment controls how much of the pump is used by controlling the polarization, and in another embodiment using the front edge of the pulse to seed subsequent portions of the pulse. Furthermore, the previous patent required that the narrow bandwidth seed be generated by double-passing the signal through a filter, whereas in the current application we use an approach that utilizes a single-pass (reflection) from the filter. These innovations are distinct from the previous patent and offer a means of generating narrow bandwidth tunable light using fewer optical components and a simpler overall design.
Disclosed herein is a tuning scheme that operates with a single crystal that has many other advantages over the tunable light sources in the prior art.